Early yesterday, the Pakistani scientist at the center of one of history's worst nuclear scandals walked out of his Islamabad villa to declare his vindication after five years of house arrest. "The judgment, by the grace of God, is good," a smiling Abdul Qadeer Khan told a throng of reporters and TV crews.

Moments earlier, a Pakistani court had ordered the release of the metallurgist who had famously admitted selling nuclear secrets to Iran, Libya and North Korea. Through years of legal limbo, Khan, 72, had never been charged, and now he never will be. "The so-called A.Q. Khan affair is a closed chapter," a Pakistani government spokesman said.

In Washington, the news sparked criticism but little surprise. It was a jarring denouement to what had been one of the most celebrated successes against nuclear weapons trafficking in decades -- a victory that has been increasingly tarnished by government failures in the aftermath of the ring's breakup.

Nearly five years after Khan's smuggling operation came to light, the international effort to prosecute its leaders is largely in shambles, yielding convictions of only a few minor participants and no significant prison time for any of them.

Meanwhile, the much-touted cooperation between the United States and its partners in the investigation of the network also is in disarray. In recent weeks, Washington has faced accusations that it withheld crucial documents from key allies and allowed its spies to run covert operations in friendly countries without permission.

Worst of all, the recent discovery of nuclear weapons blueprints on computers found in Switzerland and Dubai has prompted questions about whether the damage inflicted by the network was truly contained -- or even understood. It is possible, U.S. officials concede, that Khan and his allies shared nuclear secrets with still-unknown countries and, perhaps, terrorist groups, as well.

Khan himself remains a hero in his homeland, immune from further prosecution and free now to travel abroad as he wishes. His lenient treatment is hardly likely to deter would-be traffickers, said David Albright, a former U.N. nuclear inspector who has spent years studying the network.

"Too many network members are getting off with little punishment," said Albright, president of the Institute for Science and International Security, a Washington research group. "The busting up was handled far better than the rounding up."

Khan's international network collapsed in 2003 after U.S., British and Italian officials halted a Libya-bound ship in the Mediterranean loaded with machine parts used to make enriched uranium.

That discovery was the culmination of more than a decade of secret investigation by the CIA and other agencies of the business dealings of Khan, one of Pakistan's best-known scientists and the father of the country's nuclear weapons program.

U.S. and U.N. investigators ultimately accused Khan of heading a sophisticated network of businesses and front companies that manufactured and sold components needed to make nuclear bombs. But while the factories and shipping offices were dismantled, Khan proved to be beyond Washington's reach. Pakistan's then-President Pervez Musharraf, confronted with evidence of Khan's deeds, persuaded the scientist to make a public confession but then officially pardoned him. Khan would remain under house arrest, but Pakistani officials refused to allow him to be questioned by U.S. officials or investigators of the International Atomic Energy Agency, the U.N. nuclear watchdog.

In the years since, the restrictions on Khan were gradually lessened, and many experts believed he would be freed.

Word of Khan's release from even his modest confinement drew complaints and concern from the Obama administration.

At the State Department, spokesman Gordon K. Duguid said yesterday that Khan remains a "serious proliferation risk," and the White House asked for assurances from Pakistan that the scientist will never be allowed to resume his former work.

Pakistan's foreign ministry acknowledged the end of Khan's confinement but said Islamabad will take "all necessary measures to promote the goals of nonproliferation."

Despite such pledges, Khan's release solidified a view among some U.S. officials and weapons experts that Pakistan had not shown proper resolve in investigating the network and bringing Khan and his cohorts to justice.

Jeffrey G. Lewis, director of the Nuclear Strategy and Nonproliferation Initiative at the New America Foundation, said Khan's ability to essentially walk away from nuclear-smuggling charges "makes a mockery of our efforts to stop the spread" of nuclear weapons.

"In a spy novel, these guys would meet their fates at the murderous hands of spy agencies or each other," he said. "In real life, the guys in the Khan network do a little house arrest or maybe some pretrial detention."

Other alleged operatives got off nearly as easy, he said. "As far as I can tell, no one served a day after trial," though a German importer spent some time behind bars in pretrial detention, Lewis said.

Efforts to prosecute alleged members of the network in Switzerland touched off a series of squabbles between Swiss and U.S. officials. Swiss prosecutors accused the Bush administration of withholding critical evidence needed to put three Swiss businessmen -- a father and two brothers who worked with Khan in the 1980s and 1990s -- behind bars.

Last month, one of the brothers confirmed in a Swiss television interview that he had been working undercover for the CIA, prompting the Swiss parliament to ask why Switzerland had not been informed about covert action inside its territory.

While the investigation has yielded few arrests, it has provided disturbing insight into the sophistication of 21st-century smuggling networks and their ability to move the most sensitive weapons technology across international borders, weapons experts said. Albright, the former nuclear inspector, said it is likely that other smugglers will eventually seek to take Khan's place, and some may already have done so. If fact, he said, it would be unwise even to count Khan out.